1. Field of the Invention
The present invention relates to a recording and reproducing apparatus for recording and/or reproducing information by using a scanning probe microscope for detecting surface information of a sample by a tunnel current, force, or the like which is generated between the probe and the sample.
The information recording and reproducing apparatus of the invention incorporates each of an apparatus for performing only the reproduction of information and an apparatus for executing both of the recording and reproduction of information.
2. Related Background Art
Scanning probe microscopes (hereinafter, simply referred to as an "SPM") are classified into several types in accordance with physical properties which are detected. A number of various kinds of physical amounts such as force, light, electrostatic capacitance, and the like, like a tunnel current, an interatomic force, an electrostatic force, a magnetic force, and the like are used as targets to be detected. In all of those cases, they are common in terms of a point that the physical amount is detected by a probe having a sharp tip. The SPM has rapidly developed in recent years as a detecting method which indicates a high space resolution of 0.1 to tens of nm.
In association with the development of the SPM, many techniques about a recording and reproducing apparatus to which the principle of the SPM is applied have been proposed so far. By effectively utilizing the high space resolution of the SPM, ultramicro recording bits each having a size of one to hundreds of nm can be formed and reproduced by a stimulus such as current, electric field, force, and the like which is generated between a probe and a local region of a recording layer. A recording density can be remarkably raised.
The recording method is mainly classified into a method of giving a physical shape change to the surface of the recording layer and a method of giving a change in electrical, magnetic, chemical, or optical state to the surface of the recording layer. As the former method, there can be mentioned a method of giving a physical deformation by the probe, a method of giving a thermal deformation by a high energy beam such as a laser beam, an electron beam, or the like, a method using an electric field evaporation by a high electric field, a method of depositing micro particles, or the like. As the latter method, there can be mentioned a method using an electrical switching phenomenon which has been found out in an organic compound of the .pi. electron system or a chalcogen compound (JP-A-63-161552, JP-A-63-161553), a method using a charge accumulation in a silicon nitride film (of U.S. Pat. No. 4,575,822), a method using various kinds of oxidation reduction reactions of a transition metal oxide, viologen, styryl class compound, rare earth diphthalo cyanine, polyaniline, polythiophene, polypyrrole, metal-TCNQ charge-transfer complex, or the like, a method using a crystal phase/amorphous phase change using a crystallized glass of the vanadium oxide system, or the like.
An example of the conventional recording and reproducing apparatus will now be described with reference to FIG. 1. FIG. 1 is a schematic block diagram of an example of the conventional recording and reproducing apparatus. The apparatus shown in FIG. 1 has: a probe 102 supported to a probe fine-moving mechanism 104 through an elastic member 103; and a recording medium 101 arranged so as to face the probe 102. A tunnel current which is generated between the probe 102 and a recording layer 101a of the recording medium 101 is detected by a current detecting circuit 108. The tunnel current is supplied as a servo signal to a distance servo circuit 107, thereby controlling the distance between the probe 102 and the recording layer 101a. A recording pulse generating circuit 106 generates a voltage pulse to a portion between the probe 102 and the recording layer 101a, thereby forming a recording bit onto the recording layer 101a. On the other hand, while a bias voltage is applied to the portion between the probe 102 and the recording layer 101a by a reproduction bias generating circuit 105, the tunnel current flowing between the probe 102 and the recording layer 101a is detected by the current detecting circuit 108. By performing a predetermined process to the detected tunnel current by a reproduction signal processing circuit 113, reproduction reconstruction data is formed.
Another example of the conventional recording and reproducing apparatus will now be described with reference to FIG. 2. FIG. 2 is a schematic block diagram of another example of the conventional recording and reproducing apparatus. In the apparatus shown in FIG. 2, an interatomic force which is generated between a probe 122 and a recording layer 121a is detected by a force detecting sensor 129. The detected interatomic force is supplied as a servo signal to a distance servo circuit 127, thereby controlling the distance between the probe 122 and the recording layer 121a. A recording pulse generating circuit 126 applies a voltage pulse to a portion between the probe 122 and the recording layer 121a, thereby forming a recording bit onto the recording layer 121a. On the other hand, while a reproduction bias generating circuit 125 applies a bias voltage to the portion between the probe 122 and the recording layer 121a, a current under the bias voltage is detected by a current detecting circuit 128. By executing a predetermined process to the detected current by a reproduction signal processing circuit 133, data is thereby formed reproduction reconstruction data.
Among the above recording and reproducing apparatuses, however, the apparatus shown in FIG. 1 in which the distance between the probe and the recording layer is controlled by using the tunnel current has a problem such that noises due to a fluctuation of the surface shape of the recording layer, film thickness, film quality, or the like are mixed into the reproduction information, so that there is a case where a reproduction error occurs.
The apparatus shown in FIG. 2 in which the distance between the probe and the recording layer is controlled by the interatomic force is excellent as compared with the apparatus shown in FIG. 1 with respect to a point that the noises which are generated due to a fluctuation of the surface shape of the recording layer can be perfectly separated from the reproduction information. In the case where there is a fluctuation of the film thickness or film quality in the recording layer, however, there is a problem since there is a fear that the noises which are generated by those fluctuations are still mixed into the reproduction information.
The problems in the case where there is a fluctuation of the film thickness of the recording layer will now be described further in detail hereinbelow with reference to FIGS. 3A to 3F.
FIG. 3A is a schematic diagram of the recording layer and the probe in the case where there are fluctuations of the probe and film thickness. The diagram illustrates the recording layer such that a film thickness of each of recording regions 3 to 5 among recording regions 1 to 11 is smaller than a reference film thickness and a film thickness of each of the recording regions 9 and 10 is larger than the reference film thickness. The recording region in the ON state among the recording regions 1 to 11 is shown by a hatched region.
As shown in FIG. 3A, in a state in which a contacting state of the probe 102 and the recording layer 101a is held constant, while the probe 102 is scanned in the direction shown by an arrow A in the diagram along each of the recording regions 1 to 11, a reproduction bias voltage V.sub.0 as one of the reproducing conditions is applied as shown in FIG. 3B, so that a detection current as shown in FIG. 3C is obtained. That is, a change in detection current occurs because, in addition to a change corresponding to a conductivity change of the recording layer 101a in the recording region in the ON state, a change corresponding to the film thickness fluctuation of the recording layer 101a is mixed as noises. The detection current is output to the reproduction signal processing circuit. In the reproduction signal processing circuit, the detection current is first sampled by a reproduction timing signal shown in FIG. 3D, thereby obtaining reproduction sampling data as shown in FIG. 3E. Subsequently, by comparing the reproduction sampling data with a judgment level as another one of the reproducing conditions, reproduction reconstruction data shown in FIG. 3F is obtained. A voltage value of the judgment level signal is set to the intermediate value between the recording region output value in the ON state at a standard film thickness and a standard film quality and the recording region output value in the OFF state and is a fixed value which has previously been obtained by experiments. As shown in FIG. 3F, in the reproduction reconstruction data obtained, actually, the recording regions 3 and 5 in the OFF state are judged as regions in the ON state and the recording region 10 in the ON state is judged as a region in the OFF state, so that a reproduction error occurs.
Although the case where there is a fluctuation in the film thickness has been described above, an almost similar problem also occurs with respect to a fluctuation of the film quality. Therefore, in the conventional recording and reproducing apparatus, it is difficult to distinguish the reproduction information from the noises which are generated due to the fluctuations of the film thickness and film quality. There is a fear such that the reliability of the recording and reproducing system is remarkably lost excluding the case where no defect exists in the recording layer and the film thickness and film quality are ideally uniform.
On the other hand, the reproduction bias voltage is set in a manner such that in the standard film thickness, current changes in the ON state and OFF state can be preferably detected in a dynamic range of the current detecting system. In the case where the film thickness largely fluctuates from the standard value, however, there is a problem since there is a fear that the detected current value is deviated from the dynamic range. As an extreme example, in the case where the reproducing condition such as a reproduction bias voltage or the like exceeds a recording threshold value of a recording voltage or the like in association with a large fluctuation of the film thickness or film quality, there is also a problem such that an area other than a desired recording area is shifted to the ON state.